If two batteries refuse to charge, then this sounds like a defective bottom module (as was the case here), which is something that Fairphone support will need to address.
I’m not sure what the status of getting new spares in stock is (spares ran out when they stopped production to fix some quality control issues), nor do I know whether they have separate stock for warranty issues (which your case might be). If Fairphone give you an update, it’d be good to know!
A temporary solution could be to charge the batteries in a universal external battery charger (example - though check polarity), if the phone otherwise worked fine for the 30 mins of use you got out of the new battery (sometimes defective bottom modules cause other issues that make this workaround useless).